The introduction of integrated circuits (ICs) in the 20th century fundamentally changed the way engineers design electronic systems. In telecommunications, RF integrated circuits have performance requirements that are not readily or cheaply attainable using commercial IC technologies. These technologies are usually tailored to digital or more traditional analog designs and, consequently, are limited by the low quality (Q) factors of passive devices such as integrated inductors, capacitors, and filters. This forces engineers to design around performance limited IC devices, or to resort to hybrid integration strategies with off-chip circuit elements such as inductors, capacitors, crystals, and filters.
Today, the trend towards reduced cost, increased integration, and added functionality by solutions such as system-on-a-chip (SoC) renders off-chip components undesirable because of their impact on production quality, cost, and size. Recently, much attention has been focused on MEMS. MEMS are integrated devices or systems combining electrical and mechanical components/functionality. Their dimensions can range in size from the sub-micrometer level to the millimeter level, and there can be any number, from one, to few, to potentially thousands or millions in a particular system. Historically, MEMS devices have leveraged and extended the fabrication techniques developed for the silicon integrated circuit industry, namely lithography, doping, deposition, etching, etc. to add mechanical elements such as beams, gears, diaphragms, and springs to silicon circuits either as discrete devices or in combination with integrated silicon electronics.
Accordingly, MEMS offer the opportunity to integrate many RF sub-components on-chip, which have traditionally been implemented off-chip. This provides the microelectronics designer with a new toolset of devices and functionalities. A higher level of integration can be achieved, which translates into more functionality in the same form factor, and enables lighter, lower cost, and more portable wireless systems when applied to RF circuits and components. Compared to conventional integration-friendly devices such as integrated inductors, MEM components have the potential to offer better performance through enhanced Q-factors and lower activation power. Specifically, MEM resonators can provide flexible on-chip re-configurability and high filtering performance, and benefit from integration of the necessary signal provisioning, processing and control required from integrated electronics within the chip to which the MEM resonator is integrated.
MEM resonators have been in active development since the 1980s, and were conceptually introduced in the 1940s. In the early stages, frequency of operation of such devices were in the audio range, and integration remained a sought after mystery. More recently, developments in the field of MEMS packaging such as Y. T. Cheng, et al. [“Vacuum Packaging Technology using Localized Aluminum/Silicon-to-Glass Bonding” J. of MEMS, Vol. 11, No. 5, pp. 556-565, October 2002] and R. Legtenberg et al [“Electrically Driven Vacuum-Encapsulated Polysilicon Resonators” Sensors and Actuators A, Vol. 45, pp. 57-66, 1994] coupled with increasing developments on miniaturization have allowed high frequency operation making them an attractive alternative for RF systems.
In wireless systems, micromechanical resonators are attractive components to use as IF or image rejection filters as well as in frequency references, see for example C. Nguyen [“Microelectromechanical Devices for Wireless Communications,” Proc. IEEE Intl. Conf. on MEMS, pp. 1-7, January 1998]. MEM resonator-based filters that achieve high-Q, low insertion loss, and exhibit elevated stability when used in oscillators are hence an attractive integration alternative. In addition, MEM resonators have a resonant frequency that depends on their operating conditions such as temperature, pressure, or ambient chemical content. By capitalizing on these variations, designers can use MEM resonators to measure different physical parameters with high accuracy. MEM resonators have already been considered for use in sensing applications of gas, vibration, ultrasound, chemical and biological sensing. In other filtering applications, MEM resonators have been investigated for use in such biomedical domains as artificial cochlear implants.
In order to become a relevant and competitive technology, similar to the recent integration developments of supra-IC bulk acoustic wave (BAW) filters, see for example A. Dubois et al. [, “Integration of high-Q BAW Resonators and Filters Above ICs”, Proceedings of the IEEE Int. Solid-State Conf., Vol. 1, pp. 392-393, February 2005], MEM resonators require fabrication processes that are suited to integration with the dominant IC processes such as silicon-based CMOS. This enables the cost effective and efficient use of MEM resonators in the numerous sensing and filtering applications that are potential markets for the technology. Further, it would be advantageous if the MEM resonator fabrication process was compatible with not only silicon CMOS integration but the manufacturing of MEM resonators from materials offering enhanced performance compared to silicon such as ceramics including silicon carbide, silicon dioxide, and silicon nitride, and carbon including thin-film diamond. Such materials offering enhanced Young's modulus, high acoustic velocity, increased environmental tolerance enhanced chemical resilience and insulating structural layers for increased electrical design flexibility.